Understanding Fuel Economy: MPG vs. L/100km

When comparing vehicle efficiency across different regions, you'll encounter two primary systems: Miles Per Gallon (MPG), used predominantly in the United States and the United Kingdom, and Liters per 100 Kilometers (L/100km), which is the international standard (SI) used in Europe, Canada, and Australia.

While they both measure how much fuel a car uses, they represent the data in fundamentally different ways. MPG is a "distance-per-fuel" metric (the higher the number, the better), whereas L/100km is a "fuel-per-distance" metric (the lower the number, the better).

The Formula

Converting between these units requires specific constants because US gallons and Imperial gallons are different sizes. A US gallon is approximately 3.785 liters, while a UK gallon is approximately 4.546 liters.

US MPG to L/100km

To convert US MPG to Liters per 100km, use the following formula:

$L/100km = \frac{235.215}{MPG_{US}}$

UK (Imperial) MPG to L/100km

To convert UK MPG to Liters per 100km, use this formula:

$L/100km = \frac{282.481}{MPG_{UK}}$

L/100km to MPG

Conversely, to find MPG from L/100km:

$MPG_{US} = \frac{235.215}{L/100km}$ $MPG_{UK} = \frac{282.481}{L/100km}$

The "MPG Illusion"

One of the most important concepts in fuel economy is the non-linear relationship of MPG. Improving a car's efficiency from 10 MPG to 20 MPG saves significantly more fuel than improving from 40 MPG to 50 MPG.

This is why many engineers prefer L/100km. It shows a linear relationship with actual fuel consumption. For example, over a 100km trip, a change from 10L to 8L always saves exactly 2 liters of fuel, regardless of the starting point.

How to Use This Calculator

1. Enter your value: Type in the fuel economy number you have.
2. Select the Unit: Choose whether that number is in L/100km, US MPG, or UK MPG.
3. Advanced Options (Optional): Enter your local fuel price and your estimated annual driving distance to see your potential fuel spend.
4. Review Results: The calculator instantly provides conversions to all other units and calculates your efficiency category.

Worked Examples

Example 1: US Sedan

A car is rated at 30 US MPG. How many liters per 100km does it use?

• Formula: $235.215 / 30$
• Calculation: $235.215 / 30 = 7.84$
• Result: 7.84 L/100km

Example 2: European Hatchback

A car uses 5.5 L/100km. What is its efficiency in UK MPG?

• Formula: $282.481 / 5.5$
• Calculation: $282.481 / 5.5 = 51.36$
• Result: 51.36 UK MPG

Fuel Economy Reference Table

| Efficiency Rating | L/100km | US MPG | UK MPG | | :---------------------- | :--------- | :---------- | :---------- | | Exceptional (EV/Hybrid) | < 3.0 | > 78.4 | > 94.2 | | Excellent | 3.0 - 5.0 | 47.0 - 78.4 | 56.5 - 94.2 | | Good | 5.0 - 8.0 | 29.4 - 47.0 | 35.3 - 56.5 | | Average | 8.0 - 12.0 | 19.6 - 29.4 | 23.5 - 35.3 | | Poor | > 12.0 | < 19.6 | < 23.5 |

FAQ

Why is a UK gallon different from a US gallon?

The US gallon is based on the old English wine gallon (231 cubic inches). In 1824, the UK standardized the Imperial gallon based on the volume of 10 pounds of water (approximately 277.42 cubic inches). Consequently, UK MPG figures always look higher (better) than US MPG figures for the same vehicle.

Which unit is more accurate?

Neither is more "accurate," but L/100km is more "honest" for budgeting. If you want to know how much money you will spend on a trip, L/100km gives you a direct multiplier for the distance.

How does speed affect these numbers?

Most vehicles achieve peak fuel economy between 50-80 km/h (30-50 mph). Driving at 120 km/h (75 mph) can increase fuel consumption by as much as 20% compared to driving at 100 km/h (62 mph) due to aerodynamic drag.

Does idling affect my MPG?

Yes. When idling, you are traveling 0 miles but consuming fuel, which mathematically results in 0 MPG. In L/100km terms, idling represents an infinite consumption rate for that distance.

Can I convert km/L to L/100km?

Yes. To convert Kilometers per Liter (km/L) to L/100km, simply divide 100 by the km/L value. For example, 20 km/L is $100 / 20 = 5$ L/100km.

Is fuel economy the same as fuel efficiency?

Technically, fuel economy refers to the distance traveled per unit of fuel, while fuel efficiency is a broader term often referring to the thermal efficiency of the engine itself. However, in common conversation, they are used interchangeably.

Why do manufacturers provide different city and highway ratings?

Engines are less efficient during stop-and-go traffic (city) because of the energy required to accelerate the vehicle's mass from a standstill. On the highway, the engine can maintain a steady, optimized RPM in a higher gear, though air resistance becomes the primary efficiency thief at high speeds.